feat(sdk-coin-baby): add CreateBtcDelegation transaction

[sbplat]

Ticket: SC-1335

This pr introduces the following changes:

1. More extensible Cosmos messages

• allows Cosmos like chains to supply their own CustomMessage type
• have support to send messages other than the standard ones defined by CosmosTransactionMessage (send, delegate, undelegate, etc.)

2. Babylon CustomTransactionBuilder

• sends Babylon specific messages
  • currently the only message is CreateBtcDelegationMessage
• need to pass in a _kind attribute within the value of the send message to differentiate between the messages
  • the _kind of CreateBtcDelegationMessage is CreateBtcDelegation

[DinshawKothari]

How many Cosmos-like coins currently use CustomMessage? Is it only Babylon for now, or do we anticipate a broader need for this feature across other coins in the near future?

From my understanding, CustomMessage is specific to Babylon at the moment. Given this, it may not be necessary to propagate these changes throughout the entire Cosmos abstraction.

Instead, a more targeted approach would be to create a new class that extends the existing implementation and introduces the custom message logic there, such as:
export class CustomMessageCoin extends CosmosCoin

[sbplat]

for now, Babylon is the only coin that needs to send this custom message.

we need to modify the entire cosmos abstraction to add CustomMessage to the CosmosTransactionMessage type union. this is because we can't override the _message type from CosmosTransactionBuilder (if you have a better approach, please let me know). so even with a new class that handles the custom message logic, I think we would still need to propagate the CustomMessage type across the cosmos abstraction.